1. Field of the Invention
The present invention relates to a method for operating a membrane reactor which conducts a hydrogen formation reaction (e.g. a steam reforming reaction or a dehydrogenation reaction) using a hydrogen-selectively permeable membrane, and to a membrane reactor used in the method.
2. Related Art
In a membrane reactor wherein a hydrogen formation reaction (e.g. a steam reforming reaction or a dehydrogenation reaction) is conducted using a hydrogen-selectively permeable membrane (e.g. a Pd membrane or a Pd alloy membrane), the hydrogen formed at the hydrogen formation portion is separated and removed, whereby the conversion of the hydrogen formation reaction is made higher than the equilibrium conversion.
Use of such a membrane reactor can give a high conversion at low temperatures even for a reaction which has heretofore given a high conversion only at high temperatures; consequently, it can give a high yield at low reaction temperatures and is advantageous from the standpoints of the heat energy and reactor material required.
The reactions using a membrane reactor include the following, for example.
(a) In the dehydrogenation reaction of cyclohexane represented by the following formula: EQU C.sub.6 H.sub.12.fwdarw.C.sub.6 H.sub.6 +3H.sub.2 PA0 (b) In the steam reforming reaction of methane represented by the following formula: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 PA0 (1) The hydrogen formation reaction such as steam reforming reaction, dehydrogenation reaction or the like takes place in a catalyst. Since there is a space limitation with respect to the geometrical arrangement of the catalyst and the hydrogen-selectively permeable membrane, it is impossible to provide a hydrogen-selectively permeable membrane having an area necessary for removal of formed hydrogen, in the vicinity of the catalyst. PA0 (2) The hydrogen-transmitting amount of the hydrogen-selectively permeable membrane needs be increased. PA0 (3) The hydrogen-transmitting amount of the hydrogen-selectively permeable membrane is determined by the difference in hydrogen partial pressure between the hydrogen formation portion X and the hydrogen separation portion Y. With a high hydrogen partial pressure at the hydrogen formation portion X, however, there are the following problems: PA0 (4) In the membrane reactors of FIG. 6 and FIG. 8, the hydrogen partial pressure at the hydrogen separation portion Y is not sufficiently low. PA0 (5) In the membrane reactor shown in FIG. 7, the Ar gas used as a sweep gas is expensive and moreover, when hydrogen formation is an intended objective, the separation of Ar from hydrogen in the post-operation is difficult. PA0 the raw material gas-introducing section is connected with the hydrogen formation portion and the steam and/or carbon dioxide-introducing section is directly connected with the hydrogen separation portion, the two introducing sections being insulated from each other by sealing, and PA0 the hydrogen formation portion and the hydrogen separation portion are not separated at the formed gas-discharging section, and the gas from the hydrogen formation portion and the gas from the hydrogen separation portion are allowed to merge with each other at the discharging section.
the equilibrium conversion is higher than 90% at 600.degree. C. but is about 50% at 450.degree. C. In this case, when a membrane reactor is used, the H.sub.2 of the right side of the above formula is removed, the reaction proceeds further, and a conversion of 90% or higher can be achieved.
the equilibrium conversion is higher than 90% at 800.degree. C. but is about 50% at 500.degree. C. In this case, when a membrane reactor is used, the H.sub.2 of the right side of the above formula is removed, the reaction proceeds further, and a conversion of 90% or higher can be achieved.
There are known membrane reactors having the structures shown in FIGS. 6 to 8.
FIG. 6 shows the structure of a membrane reactor using no sweep gas. In a reaction chamber 1 is filled a catalyst 2 for hydrogen formation reaction. In the reaction chamber 1, a hydrogen-selectively permeable membrane 3 is provided in the vicinity of the catalyst 2. A raw material gas A is fed from an inlet 5 and contacts with the catalyst to form hydrogen; the formed hydrogen is transmitted from a hydrogen reaction portion X to a hydrogen separation portion Y via the hydrogen-selectively permeable membrane 3 and separated; then, the hydrogen is discharged out of the reaction chamber 1 through a hydrogen-discharging pipe 4. Meanwhile, the unreacted gas-containing waste gas from the hydrogen formation portion is discharged outside from an outlet 6. Incidentally, 7 is a sealing plate so that the raw material gas A does not enter the hydrogen separation portion Y.
In a membrane reactor having such a structure, since the hydrogen formed is separated via the hydrogen-selectively permeable membrane 3, the conversion of hydrogen formation reaction can be made higher than the equilibrium conversion.
FIG. 7 shows the structure of a membrane reactor using Ar gas as a sweep gas. In this structure, a hydrogen formation portion X and a hydrogen separation portion Y are completely separated. That is, in FIG. 7, a sweep gas inlet 8 is connected directly with the hydrogen separation portion Y; feeding of a sweep gas B into the hydrogen separation portion Y reduces the hydrogen partial pressure in the hydrogen separation portion Y and thereby a higher conversion is achieved.
FIG. 8 shows the structure of a membrane reactor which uses no sweep gas and whose hydrogen formation portion X and hydrogen separation Y are not separated at any of the raw material gas-introducing section and the formed gas-discharging section. In this structure, the amount of the raw material gas fed into the hydrogen separation portion Y and the additional proceeding of hydrogen formation reaction caused by the hydrogen transmitted through the hydrogen-selectively permeable membrane 3 are appropriately balanced, whereby the proceeding of hydrogen formation reaction can be controlled. The structure of FIG. 8 is advantageous because it has no necessity of sealing.
As stated above, membrane reactors, as compared with ordinary reactors, have advantages but need improvements as follows.
(a) the high hydrogen partial pressure at the portion X is disadvantageous for conversion because the dehydrogenation reaction or steam reforming reaction is a volume-expansion reaction, and PA1 (b) the high hydrogen partial pressure at the portion X applies a high mechanical stress to the hydrogen-selectively permeable membrane.